Through a message handling service executing on a computing environment, e-mail, calendar, and other types of messages can be exchanged from one client device to another through computer networks. In that way, e-mail messages can be transferred between client devices for users, and meetings and other events can be defined in the calendars of users. Message handling services typically rely upon standardized protocols for handling mail messages, calendar messages, and data files (e.g., images, documents, spreadsheets, etc.) attached to e-mail or calendar messages.
A message handling service can include sub-service agents, such as a message transfer agent and a mail submission agent, among others. A message submission agent can receive an e-mail message from a mail agent executing on a client device. When the message submission agent receives the e-mail message from the mail agent, it can interface with a mail transfer agent to distribute the e-mail message to other client devices. Examples of the standardized protocols that can be used for the communication of e-mail messages and attachments between a mail user agent, a message submission agent, and a message transfer agent include the Post Office Protocol (POP3), the Internet Message Access Protocol (IMAP), the Simple Mail Transfer Protocol (SMTP), and the Multipurpose Internet Mail Extensions (MIME).